This invention relates generally to capacitance measuring systems and apparatus for capacitively determining the thickness of dielectric material, such as plastic film. The present invention relates to use of a direct-measurement apparatus for automatic calibration of a capacitive-type thickness measurement apparatus capable of measuring minute variations in the thickness of thin plastic films.
In the field with which our invention is concerned, the use of capacitive measuring systems for measuring film thickness is known in the art. Also, the use of direct-measurement and contact-type measuring systems for measuring film thickness is known in the art.
One form known in the art is disclosed in U.S. Pat. No. 3,764,899, issued Oct. 9, 1973. This patent measures film thickness by passing the film through a capacitance sensor, wherein variations in film thickness are sensed as dielectric thickness variations between the capacitance members, and are detected as variations in a signal which is applied to the capacitance members.
Another form of capacitance measuring device is disclosed in U.S. Pat. No. 4,952,882, issued Aug. 28, 1990, which was submitted as an improvement in the construction of the capacitance sensor disclosed in the foregoing United States patent. This capacitance sensor assembly also measures changes in thickness of a dielectric film, such as plastic film, where the sensor components are constructed from materials having low coefficients of linear temperature expansion, resulting in a claimed measurement device which provides low measurement errors resulting from temperature effects on material expansion and dielectric changes.
The present invention is an improvement in the construction and calibration of the capacitive measuring system and related transport for serially examining plastic film material.
Measurement instruments of the type for which the invention finds utility are typically used to measure plastic film thicknesses in the range of 0.001-0.100 inch. Measurement accuracy, of both the film thickness and of thickness measurement locations, should be maintained over a range of ambient temperatures and relative humidities which are found in the typical environment where such instruments are used. A problem in prior art measurement devices has been the inability to maintain accurate measurements of film thickness and thickness measurement location over a range of ambient temperature and humidity changes. A second problem in prior art measurement devices, since the capacitive measurement technique is an indirect-measurement technique, has been the requirement to recalibrate the device for the many different types of plastic film products manufactured in a typical plant environment. Prior art devices must be recalibrated for each film type, with its different dielectric properties, to provide accurate absolute thickness information.
It is therefore an object of our invention to provide a direct-measurement sensor assembly alongside an improved capacitance sensor assembly which allows automatic calibration of the capacitance sensor using a direct-measurement method or a contact-type method. Direct measurement apparatus"" include, but are not limited to, contact-type sensor assemblies (such as mechanical, electromechanical, linear variable displacement transducers, etc.) or non-contact type sensor assemblies (such as laser, air-gauge, infra-red, nuclear, beta, x-ray, etc.) Our invention accomplishes its intended result, automatically, without the requirement of attempting to store calibration information in the memory of an embedded microprocessor or an external computer system.
Current methods of memory storage of calibration information in capacitance film thickness gauges are unreliable because capacitance changes due to ambient environment changes affect primary sensor readings over typical changes in ambient environment conditions. Also, a second problem occurs when an operator changes, intentionally or unintentionally, the gap between the upper and lower electrodes which make up the capacitance sensor, rendering the stored calibration information inaccurate. A third problem occurs when additive and/or base-material changes occur in the film types resulting in changed dielectric properties of the material, rendering currently stored calibration information inaccurate.
Our invention accomplishes its intended result, automatically, without the requirement of utilizing exotic materials in sensor component construction as was required in the aforementioned prior art patent, and without the need to store calibration information in the memory of the capacitance film thickness device.
Intregral to our invention is an improved film transport assembly for serially examining plastic film material which eliminates errors in thickness measurement location and eliminates variation in distance between individual sensor readings, while at the same time allowing accurate positioning of the film in the sensor. The invention utilizes a variable-speed, servo-controlled motor, position feedback device and motor controller assembly to accomplish these improvements. This motor control assembly is capable of starting, stopping, and accurate positioning of the film transport and film sample.
Accurate positioning of the film in the direct-measurement or contact-type and capacitance-type sensors is important because our invention first measures a specific point on the film sample with the contact-type sensor for calibration purposes, then the invention automatically moves the film sample such that the same calibration point on the film sample is positioned directly between the upper and lower capacitance sensor electrodes where the capacitive calibration reading is correlated to the contact-type calibration reading before the sample is run.
It is a second object of our invention to be capable of four modes of operation: 1) As described above, the direct-measurement sensor assembly allows automatic calibration of the capacitance sensor with thickness profile data measured using the capacitance sensor; 2) the capacitance sensor can be run as a stand-alone thickness profile data measuring system; 3) the direct-measurement sensor can be run as a stand-alone thickness profile data measuring system; and 4) running the capacitance sensor and the direct-measurement sensor simultaneously as a dual-sensor thickness profile data measuring system. The fourth mode of operation is useful, for example, in the measurement of embossed films where the direct-measurement sensor can measure peak-to-peak thickness (height) of the embossed pattern, and the capacitance sensor can measure the equivalent unembossed thickness, simultaneously.
An additional advantage of our invention is its ability to measure thickness profile information on materials where the capacitance sensor technology has weaknesses, or is not effective. Such materials include, but are not limited to, mono-layer plastic films, coextruded and laminated multi-layer plastic films, embossed materials, metallized films, heavily-colored films, paper, and like sheet materials.